Welcome to the board!

I have a 7 channel Onkyo receiver that I am using in the 5.1 mode. There is nothing wrong with using a 7.1 receiver in a 5.1 setup. You might want to audition a few 7.1 and 5.1 setups to see if you can really discern any differences. In any case, you can always go to 7.1 later on.

I have a SVS subwoofer and have been quite pleased with it. You might go to their website and take a look. They have many options and, IMO, are value priced.

The Rat. M80s, VP-150, QS8s, SVS PC 20-39+, OPPO, Onkyo 703s, Harmony 880 Sony 60" SXRD HDTV